Laser beam machining is a machining method widely used for cutting, welding, etc. Although it is a convincing tool for micro machining, the continuous beam laser machining leads to a heat affected zone, and hence it is not suitable for many applications such as in the biomedical field. The continuous laser beam adversely affects the peripheral area adjacent to the focused area of the laser beam and hence cannot produce features with very high precision and control. Ultra short laser pulses find an attractive advantage over the continuous laser beam due to the reduced effect of the heat affected zone adjacent to the focused point of the laser beam. A smaller pulse rate has less influence on the periphery of the focused beam. U.S. Pat. No. 5,656,186 by Mourou et al. teaches that reducing the pulsing cycle to the range of femto seconds results in a spot size of the machined area which is smaller than the spot size of the laser beam, since the maximum intensity is at the very center of the beam waist.
An ultra short laser pulse finds wide application in transparent material such as in optical storage devices, since the damage to the adjoining area is minimized. Methods to apply ultra short laser pulses for data storage can be carried out by directly focusing the ultra short laser pulse inside the transparent material as disclosed in U.S. Pat. No. 5,761,111 by Glezer, or by splitting the laser beam and directing the separate laser beams on to a target point as in U.S. Pat. No. 5,786,560 by Tatah et al.
Although an ultra short laser pulse is an efficient tool for micro machining precise movement of the laser beam and the resolution of the beam scanning are important factors which limit the finishing and feature size. Since the beam is focused to a very small spot size and the machined surface reflects the shape of the beam profile, filtering and processing the beam are important factors that need careful consideration. Also the ultra short laser pulse needs to be modulated so that the number of pulses striking each of the target points can be controlled and a very small material removal can be achieved. Also a better finish of the machined feature can be obtained. The shape of the beam can be modified to meet a specific application such as for machining grating lines which employs an elliptical shaped beam instead of a circular shaped beam.
The present invention takes into consideration all the above mentioned factors, such as the resolution of beam scanning, modulation of the ultra short laser pulse, filtering of the laser beam and modification of the beam shape.